For me, the fundamental difference to success was having a speech-language pathologist who understood how to treat a person who stutters, not just the stuttering.
The Stuttering Foundation, lists a great series of questions you should ask therapists when you interview them.
- How comfortable are you with treating stuttering? This is important because some speech pathologists are not comfortable working with stuttering.
- How many teens and adults who stutter have you worked with? This will help you determine whether the speech pathologist has the kind of experience you need.
- What do you think the primary goals of stuttering therapy should be for a teen/adult? This will help you decide whether the speech pathologist's ideas about goals match your own.
- What approaches do you use in speech therapy? How often is therapy scheduled? This will help you determine a schedule that works for you and helps you remain focused. 
Feel free to ask any other question you need in order to feel comfortable with your chosen therapist.
 "Why Speech Therapy", n.d., http://www.stutteringhelp.org/Default.aspx?tabid=148."